I'm planning to use Amazon S3 with Cloudfront as CDN network.
But I'm guessing that changing the url from 345345df4.cloudfront.com
to sub.domain.com
will slow the process of download
Is this correct?
It will make no difference. Clients have to make a DNS lookup to convert the name to an IP address; using a CNAME might make that slightly slower, but by tens of milliseconds. The download itself uses the IP address, and will be entirely unaltered.